The present invention concerns a helicopter rotor comprising a rigid hub, to which the base or root of each blade is connected by way of a spherical laminated stop and of a resilient-return drag brace, which is in the form of an elongate resilient-return element with integral damping means comprising elastomeric damping elements and forming a device commonly known as a frequency adaptor with integral damping means.
French Patent Application No. 7816521, filed on June 2nd, 1978 in the name of the present Applicants discloses a rotor of this kind, wherein the frequency adaptors with integral damping means are each constituted by a stack of metallic plates alternating with plates in a visco-elastic material having great powers of recovery from deformation, and are each connected, at their end remote from the hub, to the root of the corresponding blade by means of a ball-and-socket joint, and, at their inner end, to the hub so that the frequency adaptor is always slightly inclined to the corresponding blade, and so that the centre of one of the ball-and-socket joints is adjacent the flapping axis of the blade, which axis passes through the centre of the corresponding laminated spherical stop.
In the constructions of rotor which are described in the above-mentioned Patent Application, the peripheral portion of the hub, in the form of a flat ring of convex polygonal or substantially circular periphery, is traversed, in the direction of the axis of the rotor, by as many openings as there are blades, and each spherical laminated stop is fitted between the outer edge of the corresponding opening and the ends of the arms of a bifurcated element fixedly connected to the corresponding blade root. This bifurcated element is constituted either by an extension of the root of the corresponding blade, the ends of the arms of the bifurcated element being secured, for example by two bolts, to the support for the corresponding laminated spherical stop, or by a radial yoke, preferably a double yoke, the inner end of which, adjacent the hub, comprises two planar rigid plates arranged one on either side of the peripheral portion of the hub and spaced therefrom, and secured to the support for the corresponding laminated spherical stop, for example by two bolts, whereas the outer end of the radial yoke is secured to the root of the blade by two pins which are substantially at right angles to the plane of the rotor and one of which is removable to enable the corresponding blade to be "folded" in the plane of the rotor by being pivoted about the other pin.
Furthermore, in all of the forms of construction forming the subject matter of the above-mentioned Patent Application, the ball-and-socket joint, the centre of which is adjacent the flapping axis of the corresponding blade, constitutes the joint for connecting the corresponding frequency adaptor to the hub.
For the purpose of providing greater mechanical strength and of reducing the aerodynamic drag compared with that occurring with the forms of construction of the helicopter rotor in accordance with the above-mentioned Patent Application, other forms of construction which are more compact and are of simplified construction were described in an Application No. 7911585 for a first Patent of Addition, filed on May 8th, 1979.
In these forms of construction, the hub comprises a central drum which substantially forms an extension of the rotor mast and carries an upper plate and a lower plate, and one of the rigid elements of each laminated spherical stop is located between the edges of the two plates to form a crosspiece and is secured directly to these plates, whereas the other rigid element of the laminated spherical stop is connected to the root of the corresponding blade by a radial yoke, preferably a double yoke, and is recessed in that portion thereof that faces the hub, so as to permit free passage of the stop, the outer end of the corresponding frequency adaptor being connected to the yoke associated with this blade by a ball-and-socket joint the centre of which is adjacent the flapping axis of the corresponding blade.
Since the upper and lower plates of the rotor hub in these forms of construction are not recessed to receive the corresponding laminated spherical stops, and furthermore, since the edges of these plates are braced by one of the rigid elements of each of the stops, the plates of the hub offer an excellent mechanical resistance to the substantially radial tensile loads resulting from the action of the centrifugal force on the blades, as well as to the static and dynamic bending moments due to flap and drag which the blades apply to the members whereby they are connected to the hub.
However, the constructions forming the subject matter of Application No. 7911585 for a Patent of Addition like those described in Patent Application No. 7816521 wherein the bifurcated element connecting the laminated spherical stop to the blade is not formed by an extension of the root of the blade, suffer from the drawback of comprising a yoke, preferably a double yoke, interposed between the root of each blade and corresponding laminated spherical stop.
Such a yoke is generally formed by two rigid plates fixedly connected at their central zones by a cross-piece situated between them. The end portions of the rigid-plates remote from the hub accommodate, between them and with slight play, the root of the corresponding blade and carry at least two pins for securing the root of the blade to the yoke. The end portions of the rigid-plates nearer the hub, which are recessed for affording free passage to the laminated spherical stop in the arrangements disclosed in Application No. 7911585 for a Patent of Addition, are fixedly connected to the rigid element of the corresponding laminated spherical stop, which is not connected to the hub. The yoke is thus disadvantageous as regards weight and as regards radial congestion in the plane of the rotor and axial congestion in the direction of the axis of rotation of the rotor because of its presence and the fact that the roots of the blades are remote from the hub, and this has an unfavourable effect as regards aerodynamic drag of the rotor.
Furthermore, if, as is the case in the arrangement disclosed in the above-mentioned Application for a Patent of Addition, the cross-piece of the yoke is extended beyond the plates of the yoke, at the leading edge and/or the trailing edge of the corresponding blade, by a first and, possibly, a second angled attachment, shaped and arranged to receive the hinge-pin at one end of the lever for controlling the pitch of the blade and the ball-and-socket joint at the outer end of the corresponding frequency adaptor, then the yoke is of additionally complex structure and costly to produce.